We isolate the intrinsic quadrupole moment, following [1]}.
Due to the spreading of this free wave packet, its quadrupole moment has a quadratic growth with time
\(Q_{ii}(t) = 2\sigma _i^2-\sigma _j^2-\sigma _k^2+t^2\frac{\lambda _c^2}{4}\left(\frac{2}{\sigma _i^2}-\frac{1}{\sigma _j^2}-\frac{1}{\sigma _k^2}\right)\,,\qquad (i,j,k)=\text{cycle}(x,y,z)\cr Q_{xy}(t)=Q_{yz}(t)=Q_{xz}(t)=0\,.\) 
